Runt domain (Runx) transcription factors are key regulators of animal development. Each of the three mammalian Runx genes is required for the development of a major organ system, and in humans all three are associated with disease caused by uncontrolled cell proliferation. RUNXl is required for definitive hematopoiesis, and is the most frequently mutated gene in human leukemia;RUNX2 is required for osteogenesis, and its haploinsufficiency causes cleidocranial dysplasia;and RUNX3 is required for neural development in the dorsal root ganglia and for control of cell proliferation in the developing stomach, and is frequently deleted or silenced in human stomach cancer. The purpose of this grant is to define the molecular mechanisms through which Runx proteins control cell proliferation during development. Toward this end we are using sea urchin embryogenesis as a simplified model system. Unlike mammals, which have 3 Runx genes, the sea urchin Strongylocentrotus purpuratus has only a single Runx gene (SpRunt). As is the case with other Runx proteins, SpRunt forms a heterodimer with a beta subunit (SpCBFbeta). Our preliminary data show that SpRunt is required for the normal program of cell proliferation during embryogenesis and for the transcriptional activation of cyclin D. The specific aims of this grant are: (1) to further define the roles of SpRunt in cell proliferation;(2) to determine how SpRunt functions within the context of the cyclinD cis-regulatory system;and (3) to investigate how SpRunt activity is developmentally regulated by its heterodimeric partner, SpCBFbeta. These aims will be achieved by exploiting the strengths of the sea urchin embryo as a system for biochemical and molecular analyses of cell physiology, gene regulation, and development, and the availability of the S. purpuratus genome sequence, which will greatly facilitate both the cis-regulatory analysis of genes and the identification of purified proteins by mass spectrometry.